1. Field of the Invention
The invention relates to the field of directional microphones. More particularly, the invention is directed to a directional microphone system including a plurality of omni-directional microphones and a signal processor that produces a beam in a selectable direction with a selectable width.
2. Related Art
Directional microphones are characterized by their ability to reject sounds which are off-axis from the direction the microphone is pointing. The angle of off-axis rejection defines a beam which originates at the microphone, radiates outward in the direction the microphone is pointing, and diverges along the angle of off-axis rejection. A microphone that forms a beam with a narrower angle can reject off-axis sounds to a greater extent than a microphone that forms a beam with a broader angle.
The directional characteristics of a microphone can be illustrated by a directivity pattern. The directivity pattern for a microphone is created by plotting the output power of the microphone, in decibels (Db), in response to a constant sound signal which is moved in one or more complete circular patterns around the microphone. The directivity pattern produced for a directional microphone (as opposed to an omni-directional microphone) is typically a cardioid lobe. The width of the lobe is called a beam width. For example, FIG. 7 shows a directivity pattern for a typical directional microphone pointed in a look direction 707 labelled M, which forms an angle xcex8 with a reference axis X. The pattern 701 is created by plotting dB readings from microphone 703 in response to a constant sound source traversing the X-Y plane in a circular pattern 705. Directivity pattern 701 indicates that microphone 703 will be responsive to sounds with an amplitude equal to that of the constant sound source when these sounds are internal to directivity pattern 701, for example, at point 715, and will reject similar sounds when they are external to directivity pattern 701, for example, at point 717.
Directional microphones are used in numerous devices, such as speakerphones, tape recorders, and intercoms. More recently, computer-based devices have utilized directional microphones for speech recognition, teleconferencing, and in multi-media systems.
A conventional directional microphone has a fixed directivity pattern. Conventional microphones achieve directionality and noise cancellation by combinations of enclosure design, diaphragm design, and baffling. For example, an objective of an aviation microphone is to respond to only the user""s voice in an environment containing high noise ambients. In order to achieve this objective, the microphone is designed to be positioned on a boom close to the user""s mouth and in such a position that noise ambients are delivered to both sides of the diaphragm. The diaphragm is designed to be relatively stiff and therefore insensitive. The user""s speech creates a high sound pressure level on only one side of the microphone, thereby allowing a signal to be produced. A conventional studio microphone also achieves directionality through diaphragm, enclosure, and baffling design. However, in a studio environment, less extreme combinations of these three design parameters are required because the noise ambients are significantly less than in an aviation environment.
In order to effectively use such a microphone, the user must speak from within a region defined by the directivity pattern of the microphone. For example, the user cannot move too far off center from the direction the microphone is pointing because the user""s speech would be rejected by the microphone as an off-axis sound. Thus, the user must physically change the orientation of the microphone, and thereby change the angle of its directivity pattern, whenever the user moves to a location outside of the beam width. In a computer teleconference using such a conventional directional microphone, for example, a user could initially point the microphone to pick up his or her voice. If, during the conference, the user changes position, the user may then have to re-position the microphone to be directed to the user""s new position in order to enable the microphone to pick up the user""s voice.
If the microphone cannot be re-positioned, however, the user is constrained to remain in a relatively fixed location. For example, if a speakerphone with a directional microphone is mounted to a wall, and the microphone has a fixed position, the speakerphone will only respond to the user""s voice when the user is in front of the speakerphone. If the user walks around the room while speaking, such that the user is not always within the directivity pattern of the microphone, the user""s voice cuts in and out as the user crosses the path of the beam.
Occasionally, it is desirable to share a microphone among a number of users. An example is a teleconference involving multiple parties in the same room. To be effective in such an application, a conventional directional microphone must be re-positioned for each user. For example, the microphone can be continuously passed around to the person presently speaking. In such a situation, a microphone with a wider beam or an omni-directional microphone would be more effective. However, an omni-directional microphone has drawbacks in that noise from extraneous sources will not be rejected. For example, noise from an air conditioning system, a fan connected to an overhead projector, or noise from a conversation among a subset of parties may interfere with the voice of the intended speaker.
Thus, a conventional directional microphone is effective when a user is speaking from within the directivity pattern of the microphone and maintains a relatively fixed position. However, a conventional directional microphone is not effective when a user is in motion relative to the microphone or when sounds from multiple dispersed sound sources are to be sensed.
It is an object of the invention to provide a microphone system which provides the benefits of a directional microphone without the commensurate drawbacks. Specifically, it is an object of the invention to provide a microphone system that can reject noise from extraneous sources while maintaining effectiveness in a setting, such as a conference room, where the intended sound source can change position or move from one specific source to another, for example, from one person to another.
It is further an object of the invention to provide a directional microphone which includes a plurality of omni-directional microphones and a signal processor. More generally, it is an object of the invention to provide a directional microphone with a selectably steerable beam and selectably adjustable beam width.
The above and other objects of the invention are accomplished by a sound processing device including a plurality of microphones spaced apart from each other and a signal processing unit. Each microphone produces electrical signals representative of sound signals incident thereon. The signal processing unit receives the electrical signals and produces a specific direction sound signal by processing the electrical signals according to a specific sound direction. The sound processing device might further include an analog to digital converting unit which receives analog electrical signals from the microphones and produces corresponding digital electrical signals. The signal processing unit can produce the specific direction sound signal by weighting each digital signal according to the specific sound source direction and adding the weighted digital signals. A further advantageous feature of the sound processing device according to the invention includes a selector coupled to the sound processing unit for providing a signal which identifies the specific sound direction.
The above and other objects of the invention are also accomplished by a method of receiving sound information into a sound processing device from a specific direction. The method includes the steps of (1) receiving a sound wave into a plurality of microphones and sampling a signal representing the sound wave from each microphone at sampling times to obtain sampled signals, and (2) processing the sampled signals to obtain a composite signal, where the processing enhances sound waves received from the specific direction. In one embodiment according to the invention, the processing includes weighting sampled signals at each of the sampling times to produce weighted signals, the weighting being determined according to the specific direction, and summing the weighted signals to form the composite signal. In another embodiment according to the invention, the sampling times are determined separately for each microphone based on the specific direction.